Display devices such as liquid crystal displays and electroluminescence (EL) displays are provided with switching elements arranged matrix-wise on a substrate such as a glass substrate. The displays form a display pattern on an image plane by selectively driving thin film transistors (TFT) and pixel electrodes with switching elements. For example, in active matrix-type liquid crystal display devices, an array substrate is formed with TFTs, pixel electrodes and wiring which supplies signals to these parts, the substrate is disposed opposite to a counter substrate having counter electrodes, and a liquid crystal is sealed between such substrates.
As such a switching element for display devices, TFTs using silicon as the active layer have been used so far. However, a chemical vapor deposition growth (CVD) step is necessary to form the silicon thin film, which is a significant cause inhibiting reduction in production costs. Also, as the substrate, a glass substrate is usually used. However, a glass substrate is less resistant to impact and tends to break. It is proposed to use a polymer film as a substrate to cope with problems concerning the breakdown of a substrate and with the development of light-weight and flexible display devices. However, the polymer film is considerably inferior to a glass substrate in heat resistance and is unsuitable to a process of producing a silicon TFT. In light of this, studies are being made as to switching devices using, as the active layer, an organic semiconductor that can be formed at low temperatures in an inexpensive process.
Also, the mobility of carriers in an organic semiconductor is the same as or lower than that in amorphous silicon, and therefore only insufficient ON-current value is obtained. The mobility of carriers in an organic semiconductor is not sufficient to drive, particularly, a current drive-type display device such as EL displays.
There is a static induction transistor (SIT) as a switching element by which a relatively preferable ON current value is obtained even if the mobility is low. SITs are vertical transistors in which current flows in the direction of the thickness of the film and are different from usual TFTs which are horizontal types in which current flows in the direction of the sheet of the active layer. FIG. 29 is a schematic sectional view showing the general structure of a SIT. SITs generally have structures similar to those of tripods, provided on a substrate 101, in which a sheet-like gate electrode 104 formed with a large number of through-holes 108 (hereinafter called “slit” or “gate hole”) are interposed between a pair of parallel plate type electrodes each constituted of a source electrode 103 and a drain electrode 102. Semiconductor layers 105a and 105b are filled between these parallel plate electrodes and the gate hole. When voltage is applied across the gate electrode 104, a depletion layer is formed in the semiconductor layers 105a and 105b penetrating the gate hole, whereby current can be controlled.
In order to sufficiently reduce the low driving voltage and the OFF-current value in SITs using an organic semiconductor as the active layer, it is necessary to control current also in a thin depletion layer. It is therefore necessary to make the gate hole small-sized. Specifically, because organic semiconductors generally have more insufficient carrier mobility than inorganic semiconductors, the density of a dopant must be higher in order to obtain a sufficient ON-current value. When the density of a dopant is high, the depletion length of a depletion layer is shorter even under the same voltage. Therefore, it is necessary to decrease the hole diameter of the gate hole to 10 μm or less, though it depends on the concentration of a dopant in the case of SITs using an organic semiconductor as the active layer.
However, the diameter of the gate hole which can be manufactured by a lithographic process which is usually used in the production of a flat panel display such as a liquid crystal display and is carried out at relatively low costs and a low resolution is about several μms. Therefore, if it is intended to form a gate hole having a diameter of 10 μm or less precisely, high costs are necessitated. Also, there is the problem that the organic semiconductor layer is deteriorated in the step of peeling resists and metal films in the case of using a lithographic process using a current resist polymer on the organic semiconductor layer which has been already formed.
An attempt is made to use a discontinuous film that is thinly vapor-deposited as the gate electrode in SITs using an organic semiconductor as the active layer (see Kudo et al., “Synthetic Metals,” 1999, vol. 102, pp 900-903). However, the sizes of pores in the formed porous structure are not uniform and it is therefore difficult to obtain good switching characteristics. Moreover, the porous structure of the gate electrode is greatly changed according to the vapor deposition condition. It is therefore difficult to keep the characteristics of each element at a fixed level in the case of a switching element array for a display which array must be formed collectively on a substrate having a large area.
To solve this problem, a method is proposed in which a polymer film having a micro-phase isolated structure is used as an etching mask used to produce a gate electrode (see the publication of JP-A-2001-189466, JP-A means unexamined published Japanese patent application). However, it is difficult to prepare the polymer film having the micro-phase isolated structure suitable to the process. This method has also many steps and it is difficult to say that this process is an inexpensive one.
Moreover, an attempt is made to stick microparticles to a substrate. However, because this method is unsatisfactory in the stability of the stuck microparticles, and is therefore assumed to be unsuitable to utilization as alternate technologies for semiconductor lithography, it has yet to be applied to semiconductor manufacturing (see P. Hanarp et al., Colloid and Surfaces, Physicochem. Eng. Aspects 214 (2003) 23-26). There is an example in which microparticles are used as a shadow mask in a vapor deposition process to carry out film formation or etching (see C. Wedinius et al., Langmuir, 2003, 19. pp. 458-468) and another example in which a device having pores is formed (see Muraishi et al., “Shingaku Giho,” 2002, Vol. 15, pp. 13-17). It is however difficult to obtain a sufficient ON-current value by the element structure disclosed here.
On the other hand, along with the development of nanotechnologies, a method of producing fine structures having nano-size by using super-fine processing technologies has been developed. Methods of producing a resist pattern (so-called photolithography) by using electron rays or exposure to light are known as typical methods of producing dot patterns or line patterns to arrange nano-size fine structures regularly on a substrate.
FIG. 30 is a diagram showing a conventional method thereof. This method includes the following process: A resist layer 902 is applied to the surface of a substrate 901 (step I), an exposure resist layer 902a part of the resist layer 902 is exposed to a transmission light 906 through a photomask 903 (step II), then, a resist layer 902b patterned by developing is formed (step III), a catalyst metal 904 such as gold and platinum is vapor-deposited on the resist layer 902b (step IV), and then, the patterned resist layer 902b is lifted off. By this process, a patterned catalyst metal 904a is stuck to and remains on the surface of the substrate 1 (step V). After that, a fine structure 905a is formed on the substrate 901 and is made to grow into a structure like a fine structure 905b (step VI) by using a whisker crystal growth method, such as a chemical vapor phase growth (CVD) method or a molecular beam epitaxy (MBE) method, using a catalyst metal.
Whisker crystals have, for example, a diameter of about 1.0 to 100 nm and a length of about 0.5 to 100 μm, and they are also called nanoscale conductor or nanowire. One known method of growing whiskers, for example, is a VLS method utilizing a VLS (Vapor-Liquid-Solid) mechanism by using metal microparticles as a catalyst, which is a phenomenon that occurs at a vapor-phase-vapor phase or liquid phase-solid phase boundary.
A method of growing whisker crystals under control and an example in which this method is applied to the production of a sharp point small cathode are disclosed in each publication of JP-A-5-97598, JP-A-7-221344 and JP-A-2002-220300 is disclosed. In this method, an opening is formed by etching in a silicon oxide (SiO2) film formed on a substrate made of silicon and, for example, gold or gallium microparticles are stuck as a catalyst metal to the opening and then heated to grow silicon oxide (SiO2) or silicon (Si) whisker crystals. For industrial utilization of these whiskers, studies are being made concerning field emission devices, ultrasonic wave vacuum-tube amplifiers, display devices (see, for example, each publication of JP-A-2001-57146 and JP-A-2001-96499), nanoscale conductive connectors (see, for example, the publication of JP-A-2001-102381), touch sensors (see, for example, the publication of JP-A-2001-153738), micro-interconnecting circuit devices (see, for example, each publication of JP-A-2001-141633 and JP-A-2001-177052) and emitter structures (see, for example, the publication of JP-A-2001-167692).
Moreover, an example in which a metal film of about micron size on a sapphire substrate by a resist pattern and an aggregate of zinc oxide (ZnO) whisker microcrystals is grown in a direction perpendicular to the substrate (see M. H. Huang et al., “Science,” Vol. 292, 1897, 2001); an example in which a titanium oxide whisker is grown by heat treatment (see the publication of JP-A-2000-203998), and a method in which a nano-indenter that is a supermicrohardness meter is used to form a dot pattern to grow crystals from these dot patterns (the publication of JP-A-2004-12283), are known.
However, in these methods, it is difficult to arrange individual whisker crystals regularly on the substrate with maintaining each have a fine shape. These methods therefore can not be applied to electronic devices including electron emitting element.
Besides these methods in which microcrystals are grown on the substrate, other known methods of producing photonic crystals are a method in which holes formed by fine processing are used as dies to insert a thin film into these holes (see the publication of JP-A-2000-284136), a method in which a liquid polymer is cast (see the publication of JP-A-2001-91777), and a method in which a liquid polymer is filled (see the publication of JP-A-2002-277659). In recent years, a fine processing method has come to be known in which a resist is patterned using a scanning probe microscope as fine processing methods used in place of an electron ray or light exposure method (see the publication of JP-A-2000-340485).
However, these methods are unsatisfactory in the points of micronization and control of a device structure. Although an attempt is made to control a growth point, at least, devices having significantly high accuracy are required. Therefore, these methods unsuitable to industrial production, cannot be adopted.
Needle-like conductors such as whisker microcrystals are expected to be applied, particularly, to field emitters (also referred to simply as “emitter”). This is because the sharp end forms of these needle-like conductors are considered to be suitable to an emitter that emits electrons by applying a strong field to silicon or a metal tip. In addition, a fluorescent screen can be made to shine by making these emitted electrons run against the screen and therefore, these needle microcrystals are expected to be applied and developed widely in the fields of flat panel displays.
In this regard, the currently used field emitters are produced by making silicon or a metal have a sharp edge by processing it and the diameter of the end point is therefore as large as 20 to 30 nm. Electrons can only be emitted if a voltage close to or more than 100 V is applied. Also, the end points of sharp silicon and a metal are produced at a low yield, posing a large problem.
As materials used to solve this problem, studies are being made concerning the use of a carbon nanotube having a fine structure with a diameter as small as 1 to 10 nm. This reason is that the carbon nanotube is suitable for applications to field emitters. However, it is very difficult to handle this carbon nanotube and it is also difficult to selectively dispose it at necessary positions because of its too fine structure. After all, a method as primitive as mixing many nanotubes in glue, and applying the mixed glue to a substrate, is presently used. In this method, it is difficult to control the device structure exactly, and it is necessary to apply a voltage of 100 V or more to emit electrons.